This invention relates generally to optical signal processing, and more particularly it relates to semiconductor devices that can function as optical switches in which one optical beam modulates another without the need to transform either beam to an electrical signal.
Communications systems are increasingly using optical fiber as the transmission medium for propagating optical beams that carry information. Optical fiber exhibits many advantages including low loss, immunity to interference, and an extremely large bandwidth. In wavelength-division-multiplexed (WDM) systems, multiple wavelengths of light are used to establish many communication channels in a single optical fiber. The use of a number of channels at different wavelengths increases information throughput and correspondingly augments system capacity. In a typical WDM system, information-bearing optical beams or optical signal beams at the selected channel wavelengths are mixed together or multiplexed with the aid of optical couplers and launched into the fiber. At the receiver the optical signal beams are separated or demultiplexed by optical filters.
It is often necessary to transfer optical signals between optical networks operating at different wavelengths, swap channels within the same network or perform other functions requiring a particular optical signal beam to be converted and transmitted at a different wavelength. For example, a transmission system may be set up to send all information at a first wavelength to a first destination, and all information at a second wavelength to a second destination. Changing the wavelength of the optical signal beam from the first to the second wavelength therefore switches the destination of the information borne by the optical signal beam. The process of changing the information from one signal light beam to another can also be used to regenerate the signal, that is, to improve the quality of the signal.
The transfer of an optical signal beam from one channel requires both a device that can convert signal wavelengths and system architecture, incorporating the device, which can be scaled to required capacities. The prior art describes several devices and systems for such purposes.
A WDM optical system is disclosed in U.S. Pat. No. 5,504,609 to Alexander et al. This system includes complex remodulators for transferring a signal from an input wavelength to an output wavelength. Each remodulator contains a photodiode or similar means for converting an optical input signal to an electrical signal, which is then amplified, filtered, and amplified again. The resultant electrical signal is used to modulate an optical source by exploiting the electro-optical effect in a waveguide medium to create an amplitude-modulated output signal. The combination of electronic and optical elements required in the system of Alexander et al. greatly limit the net throughput of the system, and do not effectively take advantage of the increased bandwidth provided by the optical fiber. The remodulators also dissipate large amounts of power and make large arrays of switches impractical.
An all-optical wavelength converter is provided in U.S. Pat. No. 5,343,700 to Yoo. The converter acts as a nonlinear mixer to combine an input signal with a pump signal to generate an output signal of a different wavelength. Specifically, the output frequency is the difference between the pump frequency and the input frequency. The pump frequency determines the frequency shift according to the known rules of difference frequency generation (DFG). This device cannot be used to convert multiple input channels to multiple output channels selectively. Instead, a separate device is required to convert between each input frequency and output frequency, requiring a set of parallel converters operating between neighboring WDM networks. Of course, this system cannot practically be scaled to WDM systems containing large numbers of channels. Furthermore, systems based on these techniques dissipate large amounts of power and are therefore not feasible for large-scale systems.
The technique of DFG employed by the device of Yoo is used in a parametric wavelength interchanging cross-connect, described in U.S. Pat. No. 5,825,517 to Antoniades et al. The cross-connect of Antoniades et al. combines 2xc3x972 spatial optical switches with the wavelength converters of Yoo to allow arbitrary switching of signals among the channels of the WDM network. By selecting particular wavelengths of pump sources, the wavelength converters can be made to interchange signals between two channels in a single device. In other words, each wavelength converter in the cross-connect takes two input signals with wavelengths xcex2 and xcex2, and produces two output signals of wavelengths xcex2 and xcex1, transferring the information carried by input signal at wavelength xcex1 to output signal at wavelength xcex2, and vice versa. Switching between systems with more than two channels requires complicated networks of 2xc3x972 spatial switches and wavelength converters. Because each wavelength converter is limited to a few predetermined frequencies, arbitrary switching requires a series of wavelength converters, each of which has a different pump frequency. In addition, the cross-connect of Antoniades et al. uses only a single set of WDM wavelengths for both input and output signals, and does not allow for truly arbitrary switching.
Optical switches for wavelength conversion by means other than nonlinear optical frequency conversion have also been disclosed in the prior art. A number of these switches take advantage of the electroabsorption effect allowing some of these devices to operate on picosecond time scales. A high-speed electro-optical modulator is disclosed in U.S. Pat. No. 4,525,687 to Chemla et al. This semiconductor device contains a multiple quantum well structure across which an electric field is applied. The applied electric field increases absorption of light particles or photons having energies just below the band gap by the quantum-confined Stark effect (QCSE). As the electric field is increased further, the band edge shifts to lower photon energies. By carefully controlling an applied voltage, and therefore the applied electric field, optical properties of the device can be changed at will. An optical signal beam consisting of photons whose photon energy is just below the band gap of the quantum well structure is absorbed or transmitted with just a small change in the applied voltage. Because this device is an electronically-controlled optical modulator, it cannot be used alone to provide the wavelength conversion required in WDM systems. The desired result can only be produced by combining this device with a photodetector for generating the required electrical signal in response to the optical signal. As with the system of Alexander et al., the combination is complicated, incurs high power dissipation, cannot operate at the required switching speeds, and is not easily integrated into arrays.
In U.S. Pat. No. 5,339,370 Sano et al. teach an optical modulator whose light absorptive layer changes its absorption as a function of voltage applied across the modulator. Sano et al. also teach the use of the optical modulator in an optical communication system. This type of optical modulator is responsive directly to an electrical control signal and is not designed to switch optical signals in response to other optical signals. In addition, it is not suitable for fast-switching WDM networks because of its low response speed. A related type of modulator employing a multiple quantum well in which absorption is changed by an applied voltage is taught by Dutta et al. in U.S. Pat. No. 5,608,566 entitled multi-directional electro-optic switch. This switch can be used to switch optical signals between waveguides but, as in the case of the modulator of Sano et al., it is not responsive to another optical signals and its response time is too slow to be used in fast-switching WDM networks.
In U.S. Pat. No. 4,546,244 Miller teaches a nonlinear and bistable optical device with low switching energy. The device uses a means responsive to light for generating a photocurrent, a structure with a semiconductor quantum well, and a means responsive to the photocurrent for electrically controlling an optical absorption of the semiconductor quantum well region. The optical absorption of the semiconductor quantum well region varies in response to variations in the photocurrent and is used to modulate the absorption of a second light beam. Miller""s optical device can be integrated into an array and the response of the semiconductor quantum well region to the photocurrent can be used to modulate the second beam.
Unfortunately, the response times of Miller""s device are slow due to the low speed at which photocarriers propagate from the means generating the photocurrent to the means which control the optical absorption of the semiconductor quantum well region. For additional information on the physical principles governing the behavior of nonlinear bistable optical devices of this type the reader is referred to David A. B. Miller et al., xe2x80x9cThe Quantum Well Self-Electrooptic Effect Device: Optoelectronic Bistability and Oscillation, and Self-Linearized Modulationxe2x80x9d, IEEE Journal of Quantum Electronics, Vol. QE-21, No. 9, September 1985, pp. 1462-1476 and G. Livescu et al., xe2x80x9cHigh-speed absorption recovery in quantum well diodes by diffusive electrical conductionxe2x80x9d, Applied Physics Letters, Vol. 54, No. 8, 20 February 1989, pp. 748-750.
The problem of slow electrical response to the absorption of photons has been recognized in the prior art. In order to speed up the response of their optoelectronic modulator M. B. Yairi et al., xe2x80x9cHigh-speed, optically controlled surface-normal optical switch based on diffusive conductionxe2x80x9d, Applied Physics Letters, Vol. 75, No. 5, 2 August 1999, pp. 597-599 teach reliance on the mechanism of diffusive conduction to modulate absorption or reflection. The modulator taught by M. B. Yairi is designed to operate on successive light pulses and is not suitable for WDM systems.
Thus, there is a need for a device that can modulate an optical beam rapidly in response to another optical beam. Furthermore, there is a need for a switch that can be designed to perform the function of an optical cross-connect or a wavelength interchanger with response times on the order of tens of ps or less. It would be a further improvement to render such modulator or switch integrable in array structures that can operate on many optical signal beams simultaneously.
In view of the above, it is an object of the present invention to provide an optical modulator that can rapidly modulate an optical power beam in response to another optical signal beam. These two beams can have two different wavelengths or the same wavelength.
It is another object of the invention to provide an optical switch that can be used for wavelength switching in optical networks such as WDM networks.
Yet another object of the invention is to provide an optical switch that can be easily integrated into arrays including waveguides for guiding the optical beams.
These and other advantages and objects of the invention will become apparent from the ensuing description.
The objects and advantages of the invention are secured by a semiconductor device equipped with a photodetector having a low electrical capacitance Cd and a detector absorbing layer for absorbing an optical signal beam. The optical signal beam can be, e.g., an information bearing optical beam. The device is further equipped with a modulator having a low capacitance Cm and a modulator absorbing layer for absorbing an optical power beam. The modulator absorbing layer has an electric field-dependent absorption coefficient. The optical power beam can be a continuous beam that is to be modulated by the optical signal beam. The device has a low resistivity region between the photodetector and the modulator such that the electric field-dependent absorption coefficient is altered uniformly and rapidly throughout the modulator absorbing layer during absorption of the optical signal beam in the detector absorbing layer. This uniform and rapid alteration of the absorption coefficient can be achieved by maximizing a diffusive electrical conduction across the low resistivity region. In addition, the device has a high resistivity element in series with the low resistivity region for minimizing a net charge flow to and from the device.
In one convenient embodiment the low resistivity region makes up a shared layer of the photodetector and modulator. In particular, the shared layer is a lower contact layer of the photodetector and an upper cladding layer of the modulator.
In the same or another embodiment the device is equipped with voltage sources for applying electric fields to the diodes. Specifically, a first voltage source is provided for applying a detector voltage to the photodetector and a second voltage source for applying a modulator voltage to the modulator. Preferably, the first voltage source applies the detector voltage such that it reverse biases the photodetector. Likewise, the second voltage source applies the modulator voltage such that it reverse biases the modulator.
The photodetector can be made up of an upper contact layer and a lower contact layer, these two layers sandwiching the detector absorbing layer between them. The modulator can be made up of an upper cladding layer and a lower cladding layer, these two layers sandwiching the modulator absorbing layer between them. In the preferred embodiment the upper cladding layer and the lower contact layer make up a common or shared layer between the two diodes. In this case, it is this shared layer that forms the low resistivity region. It is also convenient for the upper contact layer and the lower cladding layer to have a low resistivity, such that the voltage between the upper contact layer and the lower cladding layer remains substantially constant during operation.
Under reverse bias the absorption of the optical signal by the photodetector, specifically by the detector absorbing layer of the photodetector, creates photoinduced charge or photogenerated carriers. The carriers change the detector voltage and also the modulator voltage, whereby the absorption coefficient of the modulator absorbing layer is varied and hence the absorption of the optical power beam is altered.
In yet another embodiment the lower contact layer of the photodetector and the upper cladding layer of the modulator are not shared. Instead, a low resistivity element connects the lower contact layer and the upper cladding layer. In this case the low resistivity region is made up of the lower contact layer, the upper cladding layer and the low resistivity element joining these two layers.
Depending on the application of the device of invention, the optical power beam may or may not have a different wavelength than the optical signal beam. For example, in the case of wavelength switching the two wavelengths are different. On the other hand, in the case of signal filtering or performing other signal processing functions, e.g., reduction of wavelength spread, the wavelengths will typically be the same.
In some embodiments it is advantageous that the modulator absorption layer comprise a quantum well or a number of quantum wells. Furthermore, in order to ensure better modulation, it is convenient that an optical waveguide be provided in the modulator absorption layer for guiding the optical power beam. A mode confining structure can also be incorporated in the modulator absorption layer to determine the mode or modes that are guided by the optical waveguide.
The device of the invention can operate in the transmission or reflection mode. In other words, the modulated power beam can either be transmitted or reflected. When operating in the reflection mode a reflector for reflecting the optical power beam is provided.
In accordance with the invention, the semiconductor device of the invention can be used in building an optical cross-connect. In fact, a preferred embodiment of such optical cross-connect includes an array of devices such that a number of optical signal and optical power beams can be processed simultaneously. It is also advantageous to provide waveguides for guiding the optical power beams through the devices.
The method of the invention can be practiced using semiconductor devices as described above to process optical signals in communications networks and in other applications. A detailed description of the preferred embodiments of the invention is presented below in reference to the appended drawing figures.